There are many circuit arrangements known in the art of telephony for providing a holding condition on a telephone line. Also, various prior art arrangements have been constructed for providing call privacy on single line multiextension connections. Arrangements of this type are designed to prevent another extension on the same line from becoming effectively connected to the line when someone on another extension has previously seized the phone line and has a call in progress.
In the past (and to the present day), relay circuits have been used for constructing hold circuits. Universally, hold circuits for single line telephones shunt the telephone set when the hold condition is to be established so as draw sufficient loop current through the line to cause the central office to respond as if the telephone set is still off-hook. More recently, arrangements have been constructed using semiconductor devices with voltage sensitive arrangements for establishing, maintaining, and terminating a hold condition.
More recently, "music on hold" arrangements have become popular wherein a signal, which is usually music, is injected into the line when it is placed on hold in an attempt to alleviate the boredom usually associated with waiting for a call on hold to recommence.
Since the promulgation of Part 68 of the Regulations of the Federal Communications Commission in the United States, many add-on devices to be retrofit to telephone subscriber lines have been constructed and sold. The regulations of Part 68 are designed to protect the equipment of the telephone network to which such devices are connected. Among the technical requirements of these regulations are minimum shunt impedances for devices and limits on the signal level which may be applied to the telephone line by external devices such as music on hold circuits.
Also, for devices sold directly to consumers, it is highly desirable to minimize the familiarity which the user must have with the technical aspects of telephony for proper installation. Since most privacy circuits and hold circuits using semiconductor devices are polarity sensitive, it has been common to use full wave bridge rectifier circuits as polarity guards to obviate the necessity of being able to distinguish between the tip and ring conductors. However, before the present invention, a plurality of bridge circuits to act as a polarity guard would be required to allow a user to retrofit a device, including a utilization circuit, in series with a telephone loop such as privacy circuit, and a utilization device shunting the loop such as a hold circuit. While such bridge arrangements are relatively straightforward, and could be combined in a package wherein only three terminals were brought out externally to the user, construction of such an arrangement would require the user to pay careful attention to which terminal was connected closest to the telephone and which terminal was connected to the central office.
The use of two utilization circuits in an add-on telephone circuit, one shunting the line and one in series with the line, requires that they be designed for placement at a known position in the telephone line relative to each other and the central office. If the person installing an arrangement reverses the locations, the circuit will not respond properly and normally will not operate. Consider the example of a first utilization circuit designed to be connected in series with the telephone set and a second utilization circuit shunting the subscriber loop. This is the type of arrangement used in the present invention. As described, the second utilization circuit is specifically designed to shunt the subscriber loop on the central office side of the series combination of telephone and first utilization circuit. If the arrangement is reversed, even though the polarity will be correct, the first utilization circuit will then be placed in series with the parallel combination of telephone and second utilization circuit. Thus, it is desirable to create a three terminal arrangement which may be placed in series with one of the loop conductors between a particular terminal pair, with the other terminal connected to the other conductor, which will unambiguously provide the proper polarity, and assure that the shunting utilization circuit does not have the series utilization circuit between it and the central office.
It is also known in the art that the addition of capacitance in the gate to cathode circuit of an SCR retards the speed at which commutation will occur. In other words, the rapid reapplication of voltage between the main terminals after the device has initially cut off can lead to retriggering under these circumstances. The holding circuits shown in U.S. Pat. No. 4,258,232 includes a parallel combination of a zener diode and a capacitor connected to the base of a transistor switch, which in turn connects the anode circuit of an SCR to the gate circuit for a predetermined period of time after the SCR cuts off. Such an arrangement is designed to retrigger the SCR upon reestablishment of loop current after a momentary interruption. Said patent teaches that it is impractical to capacitively store sufficient charge in a capacitor connected directly to the gate terminal of an SCR and a telephone hold circuit, and thus have included the additional devices of capacitor and transistor switch.